Parsing command-lines with usage.Options

Introduction

There is frequently a need for programs to parse a UNIX-like command line program: options preceded by - or --, sometimes followed by a parameter, followed by a list of arguments. The twisted.python.usage provides a class, Options, to facilitate such parsing.

While Python has the getopt module for doing this, it provides a very low level of abstraction for options. Twisted has a higher level of abstraction, in the class twisted.python.usage.Options. It uses Python's reflection facilities to provide an easy to use yet flexible interface to the command line. While most command line processors either force the application writer to write her own loops, or have arbitrary limitations on the command line (the most common one being not being able to have more then one instance of a specific option, thus rendering the idiom program -v -v -v impossible), Twisted allows the programmer to decide how much control she wants.

The Options class is used by subclassing. Since a lot of time it will be used in the twisted.tap package, where the local conventions require the specific options parsing class to also be called Options, it is usually imported with

from twisted.python import usage

Boolean Options

For simple boolean options, define the attribute optFlags like this:

class Options(usage.Options):

    optFlags = [["fast", "f", "Act quickly"], ["safe", "s", "Act safely"]]

optFlags should be a list of 3-lists. The first element is the long name, and will be used on the command line as --fast. The second one is the short name, and will be used on the command line as -f. The last element is a description of the flag and will be used to generate the usage information text. The long name also determines the name of the key that will be set on the Options instance. Its value will be 1 if the option was seen, 0 otherwise. Here is an example for usage:

class Options(usage.Options):

    optFlags = [
        ["fast", "f", "Act quickly"],
        ["good", "g", "Act well"],
        ["cheap", "c", "Act cheaply"]
    ]

command_line = ["-g", "--fast"]

options = Options()
try:
    options.parseOptions(command_line)
except usage.UsageError, errortext:
    print '%s: %s' % (sys.argv[0], errortext)
    print '%s: Try --help for usage details.' % (sys.argv[0])
    sys.exit(1)
if options['fast']:
    print "fast",
if options['good']:
    print "good",
if options['cheap']:
    print "cheap",
print

The above will print fast good.

Note here that Options fully supports the mapping interface. You can access it mostly just like you can access any other dict. Options are stored as mapping items in the Options instance: parameters as 'paramname': 'value' and flags as 'flagname': 1 or 0.

Inheritance, Or: How I Learned to Stop Worrying and Love the Superclass

Sometimes there is a need for several option processors with a unifying core. Perhaps you want all your commands to understand -q/--quiet means to be quiet, or something similar. On the face of it, this looks impossible: in Python, the subclass's optFlags would shadow the superclass's. However, usage.Options uses special reflection code to get all of the optFlags defined in the hierarchy. So the following:

class BaseOptions(usage.Options):

    optFlags = [["quiet", "q", None]]

class SpecificOptions(BaseOptions):

    optFlags = [
        ["fast", "f", None], ["good", "g", None], ["cheap", "c", None]
    ]

Is the same as:

class SpecificOptions(BaseOptions):

    optFlags = [
        ["quiet", "q", "Silence output"],
        ["fast", "f", "Run quickly"],
        ["good", "g", "Don't validate input"],
        ["cheap", "c", "Use cheap resources"]
    ]

Parameters

Parameters are specified using the attribute optParameters. They must be given a default. If you want to make sure you got the parameter from the command line, give a non-string default. Since the command line only has strings, this is completely reliable.

Here is an example:

from twisted.python import usage

class Options(usage.Options):

    optFlags = [
        ["fast", "f", "Run quickly"],
        ["good", "g", "Don't validate input"],
        ["cheap", "c", "Use cheap resources"]
    ]
    optParameters = [["user", "u", None, "The user name"]]

config = Options()
try:
    config.parseOptions() # When given no argument, parses sys.argv[1:]
except usage.UsageError, errortext:
    print '%s: %s' % (sys.argv[0], errortext)
    print '%s: Try --help for usage details.' % (sys.argv[0])
    sys.exit(1)

if config['user'] is not None:
    print "Hello", config['user']
print "So, you want it:"

if config['fast']:
    print "fast",
if config['good']:
    print "good",
if config['cheap']:
    print "cheap",
print

Like optFlags, optParameters works smoothly with inheritance.

Option Subcommands

It is useful, on occassion, to group a set of options together based on the logical action to which they belong. For this, the usage.Options class allows you to define a set of subcommands, each of which can provide its own usage.Options instance to handle its particular options.

Here is an example for an Options class that might parse options like those the cvs program takes

from twisted.python import usage

class ImportOptions(usage.Options):
    optParameters = [
        ['module', 'm', None, None], ['vendor', 'v', None, None],
        ['release', 'r', None]
    ]

class CheckoutOptions(usage.Options):
    optParameters = [['module', 'm', None, None], ['tag', 'r', None, None]]

class Options(usage.Options):
    subCommands = [['import', None, ImportOptions, "Do an Import"],
                   ['checkout', None, CheckoutOptions, "Do a Checkout"]]

    optParameters = [
        ['compression', 'z', 0, 'Use compression'],
        ['repository', 'r', None, 'Specify an alternate repository']
    ]

config = Options(); config.parseOptions()
if config.subCommand == 'import':
    doImport(config.subOptions)
elif config.subCommand == 'checkout':
    doCheckout(config.subOptions)

The subCommands attribute of Options directs the parser to the two other Options subclasses when the strings "import" or "checkout" are present on the command line. All options after the given command string are passed to the specified Options subclass for further parsing. Only one subcommand may be specified at a time. After parsing has completed, the Options instance has two new attributes - subCommand and subOptions - which hold the command string and the Options instance used to parse the remaining options.

Generic Code For Options

Sometimes, just setting an attribute on the basis of the options is not flexible enough. In those cases, Twisted does not even attempt to provide abstractions such as counts or lists, but rathers lets you call your own method, which will be called whenever the option is encountered.

Here is an example of counting verbosity

from twisted.python import usage

class Options(usage.Options):

    def __init__(self):
        usage.Options.__init__(self)
        self['verbosity'] = 0 # default

    def opt_verbose(self):
        self['verbosity'] = self['verbosity']+1

    def opt_quiet(self):
        self['verbosity'] = self['verbosity']-1

    opt_v = opt_verbose
    opt_q = opt_quiet

Command lines that look like command -v -v -v -v will increase verbosity to 4, while command -q -q -q will decrease verbosity to -3.

The usage.Options class knows that these are parameter-less options, since the methods do not receive an argument. Here is an example for a method with a parameter:

from twisted.python import usage

class Options(usage.Options):

    def __init__(self):
        usage.Options.__init__(self)
        self['symbols'] = []

    def opt_define(self, symbol):
        self['symbols'].append(symbol)

    opt_D = opt_define

This example is useful for the common idiom of having command -DFOO -DBAR to define symbols.

Parsing Arguments

usage.Options does not stop helping when the last parameter is gone. All the other arguments are sent into a function which should deal with them. Here is an example for a cmp like command.

from twisted.python import usage

class Options(usage.Options):

    optParameters = [["max_differences", "d", 1, None]]

    def parseArgs(self, origin, changed):
        self['origin'] = origin
        self['changed'] = changed

The command should look like command origin changed.

If you want to have a variable number of left-over arguments, just use def parseArgs(self, *args):. This is useful for commands like the UNIX cat(1).

Post Processing

Sometimes, you want to perform post processing of options to patch up inconsistencies, and the like. Here is an example:

from twisted.python import usage

class Options(usage.Options):

    optFlags = [
        ["fast", "f", "Run quickly"],
        ["good", "g", "Don't validate input"],
        ["cheap", "c", "Use cheap resources"]
    ]

    def postOptions(self):
        if self['fast'] and self['good'] and self['cheap']:
            raise usage.UsageError, "can't have it all, brother"

Type enforcement

By default, all options are handled as strings. You may want to enforce the type of your option in some specific case, the classic example being port number. Any callable can be specified in the fifth row of optParameters and will be called with the string value passed in parameter.

from twisted.python import usage

class Options(usage.Options):
    optParameters = [["shiny_integer", "s", 1, None, int]]
    optParameters = [["dummy_float", "d", 3.14159, None, float]]

Note that default values are not coerced, so you should either declare it with the good type (as above) or handle it when you use your options.

The coerce function may have a coerceDoc attribute, the content of which will be printed after the documentation of the option. It's particularly useful for reusing the function at multiple places.

def oneTwoThree(val):
    val = int(val)
    if val not in range(1, 4):
        raise ValueError("Not in range")
    return val
oneTwoThree.coerceDoc = "Must be 1, 2 or 3."

from twisted.python import usage

class Options(usage.Options):
    optParameters = [["one_choice", "o", 1, None, oneTwoThree]]

This example code will print the following help when added to your program:

$ python myprogram.py --help
Usage: myprogram [options] 
Options:
  -o, --one_choice=           [default: 0]. Must be 1, 2 or 3.